Method for processing chromium oxide-containing substances in large quantities, method for utilizing the processed substances, and products comprising the processed substances

ABSTRACT

A method of rapidly reducing chromium oxide-containing slag and the like in large quantities, in a simplified manner and without requiring high temperatures. Chromium oxides are reduced with at least one of elementary sulfur and compounds of sulfur having a valence less than 6. For an aqueous solution of the sulfur component, desirably, its sulfur content is more than 0.03% by weight. As the sulfur source, preferred is blast furnace slag (e.g., non-aged, gradually-cooled blast furnace slag) that is discharged in large quantities in the iron industry. Cr 6+  in chromium oxides is reduced in one of the following ways: (a) Chromium oxide-containing substances are sprayed with or immersed in blast furnace slag-released water that has been used in cooling blast furnace slag. (b) Chromium oxide-containing substances are mixed with blast furnace slag, and then kept in an air atmosphere. Optionally, the mixture is sprayed with blast furnace slag-released water. (c) Chromium oxide-containing substances are mixed with blast furnace slag, and then steamed. (d) Prior to the reduction treatment, chromium oxide-containing substances are preferably steamed. 100 parts by weight of reduced chromium oxide-containing compounds are mixed with from 0.1 to 90 parts by weight of sulfur-containing slag, and used in various civil engineering works.

FIELD OF THE INVENTION

The present invention relates to a method for processing chromiumoxide-containing substances, for example, stainless steel slag asdischarged in refining stainless steel, chromium slag as discharged inproducing chromium compounds such as sodium bichromate, slag of moltenwastes, sewage sludge, slag of molten sewage sludge and the like,thereby to prevent the release of Cr⁶⁺ from those substances.

BACKGROUND OF THE INVENTION

Stainless steel slag as discharged in refining stainless steel, andchromium slag as discharged in producing chromium compounds such assodium bichromate, contain chromium oxides at a level of a few %. A partof those chromium oxides is often oxidized to Cr⁶⁺, depending on theworking conditions, and Cr⁶⁺ is released from them.

Where stainless steel slag and chromium slag are used as materials inroadbeds, materials in temporary works, fillers in civil engineeringreclamation works and the like, it is indispensable that Cr⁶⁺ is notreleased from the slag.

Recently, recycling of slag as formed by melting ashes from garbageincinerators, sludge and the like in roadbeds, tiles and the like is nowunder consideration in the industry. However, the slag formed from sometypes of ashes from garbage incinerators, sludge and the like will oftenrelease Cr⁶⁺ to make it difficult to recycle the slag.

Accordingly, for safely recycling such slag in roadbeds, fillers incivil engineering reclamation works and the like, there have beenproposed some new techniques of preventing the release of Cr⁶⁺ from theslag.

For example, for preventing the release of Cr⁶⁺ from stainless steelslag, there has been disclosed a method of adding aluminum ash andmagnesia-based industrial wastes to stainless steel slag (see JapanesePatent Application Laid-Open (JP-A) No. Hei-6-171993).

When industrially carrying out the disclosed method, the intendedaluminum ash and magnesia-based industrial wastes are previouslyintroduced into a slag-receiving kiln, into which molten stainless steelslag is poured, stirred and mixed by the energy of the falling slagmelt.

In this method, however, the slag melt is highly viscous, and as to thisis different from molten steel, and it is difficult to produce ahomogeneously mixed condition by the stirring and mixing means assistedonly by the energy of the falling slag melt, resulting in the failure tocompletely stabilize all chromium oxides in the slag to thereby preventthe release of Cr⁶⁺ from the slag.

Another method was proposed, of adding a divalent S compound such as FeSor the like to molten slag resulting from decarburizing and refiningstainless steel. The resulting slag is then reduced to recover Cr in theslag. The slag is then stirred with an inert gas stream introducedthereinto, thereby making the slag have an S content of not less than0.20 % by weight so as not to release Cr⁶⁺ therefrom (see JP-AHei-8-104553).

In this method, an inert gas stream is introduced into the slag melt tohomogeneously mix the slag melt with the additive, thereby stabilizingthe chromium oxides existing in the slag. This method is effective inpreventing the release of Cr⁶ ⁴ from the slag, but requires thetroublesome operation of introducing such an inert gas stream into thereduced slag melt while using the additive for lowering the viscosity ofthe slag melt. Therefore, the method is not economical.

In addition, the method is further troublesome in that, if the additiveis added to the slag melt in the refining furnace in order to ensure thehomogeneous mixing of the slag melt with the additive, the additive willcontaminate the steel melt.

On the other hand, for preventing the release of Cr⁶⁺ from chromium slagas discharged in producing chromium compounds, a method has beenemployed of reductionally roasting the slag to thereby reduce Cr⁶⁺ toCr³⁺ for rendering the slag harmless. Economically, however, this methodis troublesome in that it requires a roasting step.

Sewage sludge is incinerated to reduce its volume prior to being usedfor reclamation. To prevent the release of Cr⁶⁺ from the incineratedashes, a method is used for controlling the air ratio duringincineration to be less than 1 (see Journal of the Drainage WorksAssociation of Japan, Vol. 38, No. 378, pp. 29-32, 1994).

However, as pointed out in this publication, it is extremely difficultto ensure the optimum operation for a variety of sludges derived fromvarious sites and having different properties, and the same applies alsoto the incineration of industrial wastes.

To reduce Cr⁶⁺ in chromium oxide-containing substances at roomtemperature, there is generally employed a method using ferrous sulfateas the reducing agent. In this method, however, it is difficult toefficiently lower the Cr⁶⁺ content of the processed substances to anenvironmentally acceptable level.

Where chromium oxide-containing substances are reduced or incineratedaccording to the methods noted above, and the thus-processed substancesare recycled in roadbeds, fillers in civil engineering reclamationworks, temporary works and the like, they are checked as to whether ornot chromium oxides were fully reduced, prior to being shipped. If theprocessed substances contain non-reduced chromium oxides, they cannot berecycled in roadbeds, fillers in civil engineering reclamation works,temporary works and the like.

OBJECTS OF THE INVENTION

An object of the present invention is to solve the problems in the priorart noted above, and to provide an industrial, simple and economicalmethod for processing chromium oxide-containing substances, such asstainless steel slag, chromium slag, slag of molten wastes, sewagesludge, slag of molten sewage sludge, soil contaminated with chromiumoxides and the like, thereby to completely prevent the release of Cr⁶⁺from those chromium oxide-containing substances.

Another object of the invention is to provide a method for processingchromium oxide-containing substances such as those mentioned abovewithin a short period of time without significantly increasing thevolume of the substances processed to attain the intended results.

Still another object of the invention is to provide a method ofrecycling chromium oxide-containing substances, and to provide materialsfor roadbeds, fillers in civil engineering reclamation works, materialsfor temporary works, materials for civil engineering and constructionworks, etc.

SUMMARY OF THE INVENTION

Having carefully studied the problems in the prior art noted above, thepresent inventors have found that, in the prior art in which the definedreduction of chromium is attained in a high-temperature condition formelting, roasting or incineration, the reduction condition is difficultto control and the defined condition is not economical.

On the basis of this finding, we have tried methods of reducing Cr⁶⁺ atroom temperature or at temperatures near room temperature. Specifically,we have made various studies and experiments for reacting variouschromium oxide-containing substances such as those mentioned above withdifferent aqueous solutions containing a variety of reducing agents,thereby to reduce Cr⁶⁺ in those substances.

As a result, we have found that, when water that contains sulfur havinga valence less than 6, especially water that contains sulfur ions andsulfur as released from the non-aged material of blast furnace slag, isused, then chromium existing in chromium oxide-containing substances canbe reduced with high reactivity in an industrial, simple and economicalmanner. On the basis of this finding, we have achieved the objects ofthe present invention.

In addition, based on this finding, we have further studied and havemade the following findings [1] to [3]:

[1] Using any of sulfur-containing substances listed below as <1> to<4>, the release of Cr⁶⁺ from chromium oxide-containing substances isextremely effectively prevented.

<1> Water that has been used for cooling blast furnace

slag (hereinafter referred to as blast furnace slag-released water).

<2> Slag containing sulfur and/or compounds of sulfur having a valenceless than 6, for example, gradually-cooled blast furnace slagspontaneously aged for less than 3 months (hereinafter referred to asnon-aged, gradually-cooled blast furnace slag), slag discharged from thepre-treatment of molten pig iron, etc.

<3> Elemental sulfur, and substances containing elemental sulfur, suchas sinter (flowers of sulfur) as recovered from sulfur-containing hotsprings and the like (hereinafter referred to as elementalsulfur-containing substances).

<4> Compounds of sulfur having a valence of less than 6, such as sodiumthiosulfate, iron sulfide, hydrogen sulfide and the like (hereinafterreferred to as compounds of sulfur with a valence of less than 6).

Generically, those substances <1> to <4> set forth above will behereinafter referred to as sulfur-containing substances.

[2] Using any of those sulfur-containing substances <1> to <4> notedabove, the release of Cr⁶⁺ from chromium oxide-containing substances isextremely effectively prevented according to any of the followingprocessing methods (a) to (d).

(a) They are kept in an aerial atmosphere.

(b) Water is sprayed over them.

(c) Chromium oxide-containing substances are immersed insulfur-containing substances.

(d) Steam is applied to them.

The above mentioned item (a) is a further surprising new finding. Therelease Cr⁶⁺ from chromium oxide-containing substances can beeffectively prevented without using such aqueous solutions containingreduction agents as used in the first experiment described on page 6,2nd and 3rd paragraph in this specification, according to the processingmethod of mixing the chromium oxide-containing substances with thesulfur-containing slag noted in [1] <2> and then simply leaving theresulting mixture in an air atmosphere.

[3] Using the sulfur-containing substances <1> to <4> optionallycombined with any divalent iron-containing substances (hereinafterreferred to as Fe(II)-containing substances), such as ferrous sulfate,produces the following advantages (1) and (2).

(1) Even for blast furnace slag-released water having a lowconcentration of reducing sulfur, of which the concentration may vary inthe water, any of the elemental sulfur-containing substances, thecompounds of sulfur with a valence less than 6 or the Fe(II)-containingsubstances may be added thereto or sprayed thereover, and chromiumoxide-containing substances to be processed are immersed in theresulting blast furnace slag-released water, whereby Cr⁶⁺ in thechromium oxide-containing substances can be reduced within a shortperiod of time, resulting in that the release of Cr⁶⁺ from thethus-processed chromium oxide-containing substances is completelyprevented.

(2) Even for slag that releases a large amount of Cr⁶⁺ according to themetal release test as set forth in Notification No. 46 of theEnvironment Agency of Japan, or for slag having a low degree ofporosity, the chromium oxide-containing substances in the slag of thattype may be mixed with any of the elemental sulfur-containingsubstances, and steam may be applied thereto, whereby the slag can beprocessed within a short period of time without increasing the volume ofthe processed slag, resulting in that the release of Cr⁶⁺ from thechromium oxide-containing substances in the thus-processed slag iscompletely prevented.

As preferred examples of the sulfur-containing substances having anoverall sulfur content of not less than 10 % by weight for sulfur and/orcompounds of sulfur having a valence less than 6, there can be mentionedelemental sulfur; elemental sulfur-containing substances such as sinterrecovered from sulfur-containing hot springs, etc.; and compounds ofsulfur with a valence less than 6 such as sodium thiosulfate, ironsulfide, hydrogen sulfide, etc. These may combined for use in theinvention.

Where blast furnace slag-released water is sprayed over chromiumoxide-containing substances to be processed therewith, the thus-sprayedsubstances are preferably kept in an aerial atmosphere to complete thereduction processing.

Also preferably, steam is applied to the chromium oxide-containingsubstances to be processed according to the invention prior to theintended reduction processing of the substances.

From 0.1 to 90 parts by weight of sulfur-containing slag may be added toand mixed with 100 parts by weight of the reduced, chromiumoxide-containing substances to give materials for roadbeds, fillers incivil engineering reclamation works, materials for temporary works, andmaterials for civil engineering and construction works.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing the relationship between the time for steamingstainless steel slag and the amount of Cr⁶⁺ released from the slag.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The invention will be described in detail hereinunder with reference toits preferred embodiments.

The first aspect of the invention is a method of reducing chromiumoxide-containing substances, such as chromium oxide-containing slag,sewage sludge, contaminated soil and the like, in large quantities andrapidly. The method of the invention requires neither particularly hightemperatures nor specific large-scale equipment.

According to the method of the invention, Cr⁶⁺ in chromiumoxide-containing substances is reduced by a reducing sulfur (at leastone of elemental sulfur and sulfur ions having a valence of less than 6,e.g. 2 or 4) at atmospheric temperature or at a temperature of up toabout 200° C. when vapor is optionally used. As the method does not useany slag melt, it does not require particularly high temperatures. Inother words, according to the method of the invention, chromiumoxide-containing substances can be reduced efficiently at lowtemperatures.

In one preferred embodiment of the method of the invention, chromiumoxide-containing substances are kept in contact with an aqueous solutionhaving an overall sulfur content greater than 0.03 % by weight (0.3g/liter) for sulfur and/or compounds of sulfur having a valence of lessthan 6 therein.

The present inventors carried out an experiment which is as follows: Areagent of CrO₃ was dissolved in pure water to prepare an aqueoussolution having a Cr⁶⁺ content of 100 mg/liter. To 10 liters of thethus-prepared aqueous solution, was added 100 g of non-aged,gradually-cooled blast furnace slag, which had been left for 1 weekafter having been cooled and ground to have a predetermined grain size,or of the gradually-cooled blast furnace slag that had beenspontaneously aged for 6 months, and was shaken for 6 hours, whereuponthe Cr⁶⁺ content of the thus-shaken aqueous solution was measured todetermine the decrease (mg) in the Cr⁶⁺ content of the solution.

The data are shown in Table 1.

In Table 1, the reducibility indicates the decrease (mg) in Cr⁶⁺ per kgof the gradually-cooled blast furnace slag used.

It is known that the non-aged, gradually-cooled blast furnace slag,depending on its grain size, has a degree of reducibility about 10 to100 times that of the gradually-cooled blast furnace slag asspontaneously aged for 6 months.

It is also known that slag having a smaller grain size has a higherdegree of reducibility.

The reason why the gradually-cooled blast furnace slag has the abilityto reduce Cr⁶⁺ is because the sulfur component existing in the slagdissolves in water to reduce Cr⁶⁺.

The present inventors carried out another experiment which is asfollows: A reagent of CrO₃ was dissolved in pure water to prepare anaqueous solution having a Cr⁶⁺ content of 100 mg/liter. To 10 liters ofthe thus-prepared aqueous solution, was added 100 g of blast furnaceslag-released water containing reducing sulfur having a valence of lessthan 6 in an amount of 0.5% by weight, and was shaken for 6 hours,whereupon the Cr⁶⁺ content of the thus-shaken aqueous solution wasmeasured to determine the decrease (mg) in the Cr⁶⁺ content of thesolution.

The data are shown in Table 2.

In Table 2, the reducibility indicates the decrease (mg) in Cr⁶⁺ per kgof the blast furnace slag-released water used.

Table 2 shows that the blast furnace slag-released water also has theability to reduce Cr⁶⁺, which is nearly comparable to that of thenon-aged, gradually-cooled blast furnace slag in terms of the value perthe unit weight.

In view of this, we clarified the mechanism of action of those reducingsubstances. Table 3 shows the concentrations of sulfurs of differenttypes in the gradually-cooled blast furnace slag as used for reducingCr⁶⁺, before and after the reducing treatment using it.

Comparing the sulfur concentrations before and after the reducingtreatment, it is seen that the concentrations of sulfurs except S²⁻ werelowered after the reducing treatment.

In addition, we measured the concentrations of sulfurs of differenttypes in the reduced aqueous solutions, and found that most sulfurstherein were in the form of SO₄ ²⁻.

Accordingly, it is believed that S⁰, S₂O₃ ²⁻ and SO₄ ²⁻ in thegradually-cooled blast furnace slag dissolve in water, and that ofthese, S⁰ and S₂O₃ ²⁻ are oxidized to SO₄ ²⁻ while reducing Cr⁶⁺.

The reason why the ability to reduce Cr⁶⁺ in the slag powder having asmaller grain size is higher is because the slag powder having a smallergrain size has a larger specific surface area so that the S⁰ and S₂O₃ ²⁻therein are easier to dissolve in water.

The first aspect of the present invention has been completed on thebasis of the findings noted above.

According to the invention, when an aqueous solution containing sulfurand/or compounds of sulfur having a valence less than 6 is contactedwith chromium oxide-containing substances, it reduces chromium to stablechromium compounds of Cr(OH)₃ and the like, whereby the release ofchromium oxides from the chromium oxide-containing substances isprevented.

Even if some chromium ions are released from chromium oxide-containingsubstances and dissolve to the aqueous solution, they are reduced in theaqueous solution so that the release of Cr⁶⁺ from the chromiumoxide-containing substances is prevented.

In the preferred embodiment of the first aspect of the invention, theoverall sulfur content of the aqueous solution that contains sulfurand/or compounds of sulfur having a valence less than 6 is desirablylarger than 0.03% by weight.

This is because an aqueous solution having an overall sulfur content ofnot larger than 0.03% by weight for sulfur and/or compounds of sulfurhaving a valance less than 6 therein takes a few months or longer toreduce chromium oxide-containing substances, and is therefore difficultto use in industrial processes.

As sulfur and/or compounds of sulfur having a valence less than 6, andtheir sources, which are used in the first aspect of the invention,there are preferred one or more sulfur-containing substances that may beselected from the following items <1> to <4>, which, however, are notlimitative. Apart from those mentioned below, any other substancescontaining sulfur and/or compounds of sulfur having a valence less than6 are employable herein. In other words, the sources of reducing sulfursto be used in the invention are not specifically defined, but, forexample, may be blast furnace slag that is discharged in largequantities in the iron and steel industry.

<1> Water as used for cooling blast furnace slag (blast furnaceslag-released water).

<2> Slag containing sulfur and/or compounds of sulfur having a valenceless than 6, for example, gradually-cooled blast furnace slag asspontaneously aged for less than 3 months (non-aged, gradually-cooledblast furnace slag), slag as discharged in the pre-treatment of pig ironmelt, etc.

<3>Elemental sulfur, and substances containing elemental sulfur, such assinter as recovered from sulfur-containing hot springs and the like(elemental sulfur-containing substances).

<4> Compounds of sulfur having a valence less than 6, such as sodiumthiosulfate, iron sulfide, hydrogen sulfide and the like (compounds ofsulfur with a valence less than 6).

TABLE 1 Reducibility Type of Slag Grain Size (mm) (mg-Cr⁶⁺/kg-slag)Non-aged, 13.2.-4.75  1220 Gradually-cooled  4.75-0.425 1460 BlastFurnace 0.425-0.075 1720 Slag(*) <0.075 2040 Gradually-cooled 132.-4.7515 Blast Furnace  4.75-0.425 25 Slag as 0.425-0.075 85 spontaneously<0.075 263 aged for 6 months Note: (*)Slag as left for 1 week afterhaving been cooled and ground.

TABLE 2 Reducibility (mg-Cr⁶⁺/kg-blast furnace slag- Type ofSlag-released Water Grain Size (mm) released slag) Blast FurnaceSlag-released — 1340 Water(*) Note: (*)Reducing sulfur content = 0.05%by weight

TABLE 3 Before Reducing After Reducing Treatment (wt. %) Treatment (wt.%) S as S²⁻ 0.04 0.04 S as S⁰ 0.22 <0.005 S as S₂O₃ ²⁻ 0.31 0.02 S asSO₄ ²⁻ 0.52 0.19

Based on the first aspect of the invention noted above, the presentinventors have further studied and obtained the following findings:

[1] Using any of sulfur-containing substances of the following items <1>to <4>, the release of Cr⁶⁺ from chromium oxide-containing substances isextremely effectively prevented.

<1> Water as used for cooling blast furnace slag (blast furnaceslag-released water).

<2> Slag containing sulfur and/or compounds of sulfur having a valenceless than 6, for example, gradually-cooled blast furnace slag asspontaneously aged for less than 3 months (non-aged, gradually-cooledblast furnace slag), slag as discharged in the pre-treatment of pig ironmelt, etc.

<3> Elemental sulfur, and sinter as recovered from sulfur-containing hotsprings (elemental sulfur-containing substances).

<4> Compounds of sulfur having a valence less than 6, such as sodiumthiosulfate, iron sulfide, hydrogen sulfide and the like (compounds ofsulfur with a valence less than 6).

[2] Using any of the sulfur-containing substances <1>to <4> noted above,the release of Cr⁶⁺ from chromium oxide-containing substances isextremely effectively prevented according to any of the followingprocessing methods (1) to (4).

(1) A method of keeping them in an aerial atmosphere.

Chromium oxide-containing substances are mixed with slag that containssulfur and/or compounds of sulfur having a valence less than 6, and theresulting mixture is left in an aerial atmosphere.

(2) A method of spraying water over them.

(a) Blast furnace slag-released water is sprayed over chromiumoxide-containing substances.

(b) Chromium oxide-containing substances are mixed with slag thatcontains sulfur and/or compounds of sulfur having a valence less than 6,and water and/or blast furnace slag-released water is sprayed over theresulting mixture.

(3) A method of immersing chromium oxide-containing substances insulfur-containing substances.

(a) Chromium oxide-containing substances are immersed in blast furnaceslag-released water.

(b) Chromium oxide-containing substances are immersed in blast furnaceslag-released water to which are added substances that contain sulfurand/or compounds of sulfur having a valence less than 6.

(4) A method of applying steam to them.

(a) Chromium oxide-containing substances are mixed with substances thatcontain sulfur and/or compounds of sulfur having a valence less than 6,and steam is applied to the resulting mixture.

(b) Chromium oxide-containing substances are mixed with slag thatcontains sulfur and/or compounds of sulfur having a valence less than 6,and steam is applied to the resulting mixture.

(c) Chromium oxide-containing substances are mixed with slag thatcontains sulfur and/or compounds of sulfur having a valence less than 6along with substances that contain sulfur and/or compounds of sulfurhaving a valence less than 6, and steam is applied to the resultingmixture.

[3] Using the sulfur-containing substances <1> to <4> optionallycombined with any of divalent iron-containing substances(Fe(II)-containing substances), such as ferrous sulfate, produces thefollowing advantages (1) and (2).

(1) Even for the blast furnace slag-released water <1> having a smallconcentration of reducing sulfur, of which the concentration may vary inthe water <1>, substances that contain sulfur and/or compounds of sulfurhaving a valence less than 6 may be added thereto, and chromiumoxide-containing substances to be processed are immersed in theresulting blast furnace slag-released water, whereby Cr⁶⁺ in thechromium oxide-containing substances can be reduced within a shortperiod of time, resulting in that the release of Cr⁶⁺ from thethus-processed chromium oxide-containing substances is completelyprevented.

(2) Even for slag that releases a large amount of Cr⁶⁺ in the metalrelease test set forth in the Notification No. 46 of the EnvironmentAgency of Japan, or for slag having a low degree of porosity, thechromium oxide-containing substances in the slag of that type, or boththe chromium oxide-containing substances therein, and slag that containssulfur and/or compounds of sulfur having a valence less than 6 may bemixed with substances that contain sulfur and/or compounds of sulfurhaving a valence less than 6, and steam may be applied thereto, wherebythe slag that contains chromium oxide-containing substances can beprocessed within a short period of time without increasing the volume ofthe processed slag, resulting in that the release of Cr⁶⁺ from thechromium oxide-containing substances in the thus-processed slag iscompletely prevented. Accordingly, in another preferred embodiment ofthe first aspect of the invention, chromium oxide-containing substancesare mixed with slag containing sulfur and/or compounds of sulfur havinga valence less than 6, and the resulting mixture is kept in an aerialatmosphere.

In this embodiment, non-aged, gradually-cooled blast furnace slag and/orslag discharged from pre-treatment of molten pig iron are preferred asthe slag containing sulfur and/or compounds of sulfur having a valenceless than 6, and more preferred is non-aged, gradually-cooled blastfurnace slag.

In still another preferred embodiment of the first aspect of theinvention, blast furnace slag-released water, which is water as used forcooling blast furnace slag, is sprayed over chromium oxide-containingsubstances.

In still another preferred embodiment thereof, chromium oxide-containingsubstances are mixed with slag containing sulfur and/or compounds ofsulfur having a valence less than 6, and water and/or blast furnaceslag-released water, which is water as used for cooling blast furnaceslag, is sprayed over the resulting mixture.

In this embodiment, non-aged, gradually-cooled blast furnace slag and/orslag as discharged from the pre-treatment of molten pig iron arepreferred as the slag containing sulfur and/or compounds of sulfurhaving a valence less than 6, and more preferred is non-aged,gradually-cooled blast furnace slag.

In still another preferred embodiment of the first aspect of theinvention, chromium oxide-containing substances are immersed in blastfurnace slag-released water, which is water that has been used forcooling blast furnace slag.

In still another preferred embodiment thereof, chromium oxide-containingsubstances are immersed in blast furnace slag-released water to whichare added substances containing sulfur and/or compounds of sulfur havinga valence less than 6.

In those embodiments, preferred are sulfur-containing substances havingan overall sulfur content of not less than 10% by weight for sulfurand/or compounds of sulfur having a valence less than 6 therein, as thesubstances containing sulfur and/or compounds of sulfur having a valenceless than 6.

The sulfur-containing substances having an overall sulfur content of notless than 10% by weight for sulfur and/or compounds of sulfur having avalence less than 6 therein for use in those embodiments are notspecifically defined, but preferred are, for example, elemental sulfur;substances containing elemental sulfur such as sinter as recovered fromsulfur-containing hot springs (elemental sulfur-containing substances);and compounds of sulfur having a valence less than 6, such as sodiumthiosulfate, iron sulfide, hydrogen sulfide and the like (compounds ofsulfur with a valence less than 6). Two or more of these may be used incombination.

As the substances containing compounds of sulfur having a valence lessthan 6, preferred are those that hardly react with oxygen dissolved inwater.

This is because, if sulfur-containing substances that easily react withoxygen dissolved in water are used, they will react with oxygen in waterprior to acting on chromium oxide-containing substances, thereby losingtheir ability to completely reduce Cr⁶⁺ in the chromium oxide-containingsubstances.

The reducing sulfur concentration in blast furnace slag-released watervaries, depending on the season and the weather, and may be often notmore than 0.03% by weight. If blast furnace slag-released water havingsuch a low reducing sulfur concentration is used, it takes a long timeto reduce Cr⁶⁺ in chromium oxide-containing substances.

In view of this, the present inventors have further studied and madevarious experiments in order to find an effective method capable ofprocessing chromium oxide-containing substances within a short period oftime even with blast furnace slag-released water having a low reducingsulfur concentration of not more than 0.03% by weight, therebycompletely to prevent the release of Cr⁶⁺ from the thus-processedchromium oxide-containing substances.

As a result, we have found that, when substances containing sulfurand/or compounds of sulfur having a valence less than 6 are added toblast furnace slag-released water and chromium oxide-containingsubstances are immersed in the resulting mixture, then the release ofCr⁶⁺ from the thus-processed chromium oxide-containing substances can becompletely prevented, even though the blast furnace slag-released waterused has a low reducing sulfur concentration of not more than 0.03% byweight and even though the processing is effected within a short periodof time.

Accordingly, even for blast furnace slag-released water having a lowreducing sulfur concentration of not more than 0.03% by weight, ifsubstances containing sulfur and/or compounds of sulfur having a valenceless than 6 are added thereto, chromium oxide-containing substances maybe immersed in the resulting mixture only within a short period of timethereby completely to prevent the release of Cr⁶⁺ from thethus-processed chromium oxide-containing substances.

The reducing sulfur concentration in blast furnace slag-released wateras referred to herein is obtained by subtracting the sulfur content ofSO₄ ²⁻ from the overall sulfur concentration in the water.

According to the preferred embodiment noted above, the substancescontaining sulfur and/or compounds of sulfur having a valence less than6 added to blast furnace slag-released water shall control the reducingsulfur concentration in the thus-mixed water, with which, therefore,Cr⁶⁺ in the chromium oxide-containing substances processed is rapidlyreduced.

In this preferred embodiment, the amount of the substances containingsulfur and/or compounds of sulfur having a valence less than 6 to beadded to blast furnace slag-released water is preferably so controlledthat the reducing sulfur concentration in the blast furnaceslag-released water containing those substances is greater than 0.03% byweight.

This is because, if the reducing sulfur concentration in the blastfurnace slag-released water used is not more than 0.03% by weight, it isdifficult to completely reduce Cr⁶⁺ in chromium oxide-containingsubstances with the water within a short period of time.

Also preferably, divalent iron-containing substances such as ferroussulfate, ferrous chloride and the like (Fe(II)-containing substances)may be added to blast furnace slag-released water for use in theinvention.

Adding divalent iron ions to blast furnace slag-released water ensuresmore rapid and more complete reduction of Cr⁶⁺ in chromiumoxide-containing substances as processed with the divalentiron-containing water.

In still another preferred embodiment of the first aspect of theinvention, chromium oxide-containing substances are mixed withsubstances containing sulfur and/or compounds of sulfur having a valenceless than 6, and steam is applied to the resulting mixture.

The present inventors have further studied and made various experimentsin order to find an industrial, simple, economical and effective methodcapable of processing slag that releases Cr⁺⁶ in an amount of 10mg/liter or more in the metal release test set forth in the NotificationNo. 46 of the Environment Agency of Japan, or slag having a degree ofporosity of about 5% or lower, within a short period of time withoutincreasing the volume of the processed slag, thereby completely toprevent the release of Cr⁶⁺ from the thus-processed chromiumoxide-containing substances in the slag.

As a result, we have found that, when the slag of that type containingtherein chromium oxide-containing substances is mixed with substancescontaining sulfur and/or compounds of sulfur having a valence less than6, and steam is applied to the resulting mixture, then the slag can beprocessed within a short period of time without increasing the volume ofthe processed slag, resulting in that the release of Cr⁶⁺ from thechromium oxide-containing substances in the thus-processed slag iscompletely prevented.

In this embodiment, preferred are sulfur-containing substances having anoverall sulfur content of not less than 10% by weight for sulfur and/orcompounds of sulfur having a valence less than 6 therein, as thesubstances containing sulfur and/or compounds of sulfur having a valenceless than 6.

This is because the amount to be added of sulfur-containing substanceshaving a larger overall sulfur content of sulfur and/or compounds ofsulfur having a valence less than 6, relative to the chromiumoxide-containing substances existing in the slag to be processed, may besmaller, resulting in that the increase in the volume of the processedslag may be minimized, that the amount of the steam to be used may bedecreased, and that the processed slag is easy to transport.

The sulfur-containing substances having an overall sulfur content of notless than 10% by weight of sulfur and/or compounds of sulfur having avalence less than 6 therein for use in this embodiment are notspecifically defined, but preferred are, for example, elemental sulfur;substances containing elemental sulfur such as sinter as recovered fromsulfur-containing hot springs (elemental sulfur-containing substances);and compounds of sulfur having a valence less than 6, such as sodiumthiosulfate, iron sulfide and the like (compounds of sulfur with avalence less than 6). Two or more of these may be used in combination.

In still another preferred embodiment of the invention, chromiumoxide-containing substances are mixed with slag containing sulfur and/orcompounds of sulfur having a valence less than 6, and steam is appliedto the resulting mixture.

In this embodiment, preferred are non-aged, gradually-cooled blastfurnace slag and/or slag discharged from the pre-treatment of molten pigiron, as the slag containing sulfur and/or compounds of sulfur having avalence less than 6; and more preferred is non-aged, gradually-cooledblast furnace slag.

In still another preferred embodiment of the invention, chromiumoxide-containing substances are mixed with slag containing sulfur and/orcompounds of sulfur having a valence less than 6 and also withsulfur-containing substances containing sulfur and/or compounds ofsulfur having a valence less than 6, and steam is applied to theresulting mixture.

In this embodiment, preferred are non-aged, gradually-cooled blastfurnace slag and/or slag as discharged from the pre-treatment of moltenpig iron melt, as the slag containing sulfur and/or compounds of sulfurhaving a valence less than 6; and more preferred is non-aged,gradually-cooled blast furnace slag.

In this, also preferred are sulfur-containing substances having anoverall sulfur content of not less than 10% by weight for sulfur and/orcompounds of sulfur having a valence less than 6, as the substancescontaining sulfur and/or compounds of sulfur having a valence less than6.

This is because the amount to be added of sulfur-containing substanceshaving a larger overall sulfur content for sulfur and/or compounds ofsulfur having a valence less than 6 may be smaller, resulting in thatthe increase in the volume of the processed chromium oxide-containingsubstances may be minimized, that the amount of the steam to be used maybe decreased, and that the processed chromium oxide-containingsubstances are easy to transport.

The sulfur-containing substances having an overall sulfur content of notless than 10% by weight for sulfur and/or compounds of sulfur having avalence less than 6 therein for use in this embodiment are notspecifically defined, but preferred are, for example, elemental sulfur;substances containing elemental sulfur such as sinter as recovered fromsulfur-containing hot springs (elemental sulfur-containing substances);and compounds of sulfur having a valence less than 6, such as sodiumthiosulfate, iron sulfide and the like (compounds of sulfur with avalence less than 6). Two or more of these may be used in combination.

In this embodiment, the steam applied acts on chromium oxide-containingsubstances to thereby make Cr⁶⁺ in those substances easily releasable,or that is, make it easily reducible. In this, in addition, sulfur andcompounds of sulfur having a valence less than 6 are released from theslag that contains sulfur and/or compounds of sulfur having a valenceless than 6, such as gradually-cooled blast furnace slag, and from thesulfur-containing substances that contain sulfur and/or compounds ofsulfur having a valence less than 6, and those sulfur and sulfurcompounds are oxidized while reducing Cr⁺⁶ in the chromiumoxide-containing substances to Cr³⁺.

Therefore, according to this embodiment in which are combined slag thatcontains sulfur and/or compounds of sulfur having a valence less than 6,and sulfur-containing substances that contain sulfur and/or compounds ofsulfur having a valence less than 6, chromium oxide-containing compoundscan be processed within a much shorter period of time even when theamount of the slag added is reduced.

The reducing mechanism in this embodiment is as follows:

When steam is applied to chromium oxide-containing substances having aporosity of 5% or smaller and a grain size of 40 mm or smaller, it takesabout 1 day to make 50% of Cr⁶⁺ in the substances easily releasable,about 2 days to make 80% of Cr⁶⁺ in the substances easily releasable,and about 3 days to make 100% of Cr⁶⁺ in the substances easilyreleasable.

On the other hand, when steam is applied to non-aged, gradually-cooledblast furnace slag, the amount dissolving in water of sulfur with avalence less than 6 in the slag is proportional to the period of time ofthe steam application, and it takes about 10 days before the entireamount of the sulfur of that type dissolves in water.

On the other hand, the elemental sulfur existing in the slag dissolvesin water within about 3 days.

Accordingly, where only non-aged, gradually-cooled blast furnace slag isadded to chromium oxide-containing substances, 100% of Cr⁶⁺ in thesubstances is made easily releasable, or that is, easily reduciblewithin about 3 days, whilst the releasing speed of sulfur with a valenceless than 6 from the slag is low and the sulfur of that type takes 3days or longer to be released from the slag.

Needlesstosay, if an excess amount of non-aged, gradually-cooled blastfurnace slag is added, the processing of chromium oxide-containingsubstances may be completed within 3 days. However, adding such anexcess amount of slag is troublesome in that the volume of the processedsubstances increases, that the amount of the steam to be appliedincreases, and that the processed substances are not easy to transport.

On the other hand, if, for example, elemental sulfur only is added asthe sulfur-containing substance, it dissolves in water within about 3days, and its ability to reduce Cr⁶⁺ lasts for about 3 days.

On the other hand, it takes about 3 days to make 100% of Cr⁶⁺ inchromium oxide-containing substances having a porosity of 5% or smaller,to which is applied steam, easily releasable, or that is, easilyreducible. Therefore, if elemental sulfur only is added to thesubstances, its ability to reduce Cr⁶⁺ in the substances is lost beforeit acts on the substances to completely reduce Cr⁶⁺ therein.

Taking these facts into consideration, the present inventors tried thecombination of non-aged, gradually-cooled blast furnace slag andsubstances that contains sulfur and/or compounds of sulfur having avalence less than 6, to which was applied steam, resulting in that Cr⁶⁺in the chromium oxide-containing substances was completely reducedwithin a short period of time without significantly increasing thevolume of the processed substances.

Specifically, in this embodiment, Cr⁶⁺ in chromium oxide-containingsubstances is reduced essentially by the substances that contain sulfurand/or compounds of sulfur having a valence less than 6, such aselemental sulfur, sinter and the like, in the initial stage of 1 to 2days while steam is applied to the system, and is reduced essentially bythe slag that contains sulfur and/or compounds of sulfur having avalence less than 6, such as non-aged, gradually-cooled blast furnaceslag and the like, in the final stage of 2 to 3 days while steam isapplied thereto.

There follow (I) preferred examples of the slag that contains sulfurand/or compounds of sulfur having a valence less than 6, and those ofthe blast furnace slag-released water, (II) preferred ranges of theamount to be added of the slag that contains sulfur and/or compounds ofsulfur having a valence less than 6, the amount of the blast furnaceslag-released water to be sprayed, and the reducing sulfur concentrationin the blast furnace slag-released water, and (III) preferred modes ofprocessing chromium oxide-containing substances, which are for theembodiments of the first aspect of the invention mentioned hereinabove.

(I) Preferred examples of slag that contains sulfur and/or compounds ofsulfur having a valence less than 6, and those of blast furnaceslag-released water:

As the slag that contains sulfur and/or compounds of sulfur having avalence less than 6, which is used in the invention, preferred arenon-aged, gradually-cooled blast furnace slag and/or slag as dischargedin the pre-treatment of molten pig iron, and more preferred is non-aged,gradually-cooled blast furnace slag. Non-aged, gradually-cooled blastfurnace slag:

The non-aged, gradually-cooled blast furnace slag for use in theinvention is slag that gives yellowish cloudy water, or that is, slagthat colors in the color identification test defined in JIS A5015,Appendix 1. Generally, this is gradually-cooled blast furnace slag asspontaneously aged for less than 3 months.

More preferred is non-aged, gradually-cooled blast furnace slag justhaving been cooled and ground, or that is, within 1 week after thegrinding.

The grain size of the non-aged, gradually-cooled blast furnace slag isnot specifically defined, but a fine powdery one having a smaller grainsize is better. This is because such fine powdery slag having a smallergrain size has a higher ability to reduce chromium oxides.

More preferred is non-aged, gradually-cooled blast furnace slag, ofwhich the grain size is so defined that the slag is, after having beenused for reducing chromium oxides, still effectively usable inindustrial products.

In this connection, for example, where stainless steel slag is reducedand is recycled as a material in roadbeds, if fine-powdery, non-aged,gradually-cooled blast furnace slag is used too much as the reducingagent for the stainless steel slag, the reduced slag would no longersatisfy the requirement of the grain size distribution for roadbeds asstipulated in JIS A5015.

Accordingly, in this case, it is desirable to use non-aged,gradually-cooled blast furnace slag of which the grain size satisfiesthe requirement of the grain size distribution for roadbeds. Slagdischarged in the pre-treatment of molten pig iron:

The slag as discharged in the pre-treatment of molten pig iron for usein the invention is one to be discharged in the pre-treatment step ofdesulfurizing and dephosphorizing molten pig iron before the molten pigiron is transferred into a converter. Blast furnace slag-released water:

The blast furnace slag-released water for use in the invention ispreferably one that has been sprayed over non-aged, gradually-cooledblast furnace slag, or one that has been sprayed over hot slag justhaving been discharged from a blast furnace.

Specifically, as the blast furnace slag-released water for use in theinvention, preferred is one that has been sprayed over gradually-cooledblast furnace slag that has been spontaneously aged generally for lessthan 3 months, or one that has been sprayed over hot slag that has justbeen discharged from a blast furnace.

Where the blast furnace slag-released water of that type is used, theoverall sulfur content of the water, which results from sulfur and/orcompounds of sulfur having a valence less than 6, is preferably greaterthan 0.03% by weight.

This is because, as mentioned hereinabove, blast furnace slag-releasedwater having an overall sulfur content of not more than 0.03% by weightfor sulfur and/or compounds of sulfur having a valence less than 6 takesa few months or longer to reduce chromium oxide-containing substances,and is therefore difficult to use in industrial processes.

(II) Preferred ranges of the amount to be added of slag that containssulfur and/or compounds of sulfur having a valence less than 6, theamount of blast furnace slag-released water to be sprayed, and thereducing sulfur concentration in blast furnace slag-released water, areas follows:

The amount to be added, of the slag that contains sulfur and/orcompounds of sulfur having a valence less than 6, the amount of theblast furnace slag-released water to be sprayed, and the reducing sulfurconcentration in the blast furnace slag-released water are notspecifically defined, but are preferably as follows:

[1] In the embodiment of keeping the reduction system in an aerialatmosphere, in the embodiment of spraying water over the reductionsystem, and in the embodiment of Introducing steam into the reductionsystem:

In those embodiments, where the typical blast furnace slag as in Table 3is used as the slag that contains sulfur and/or compounds of sulfurhaving a valence less than 6, such as non-aged, gradually-cooled blastfurnace slag or the like, it is desirable that the amount of the slag tobe added to the reduction system (hereinafter referred to as slagamount), the amount of the blast furnace slag-released water to besprayed over the system, and the amount to be added thereto of thesubstances that contain sulfur and/or compounds of sulfur having avalence less than 6 (e.g., elemental sulfur-containing substances,sulfur-containing substances with a valence less than 6) satisfy thefollowing requirements (1) and (2):

10≧A≧0.1  (1)

wherein;

A=[amount of slag added (wt.pts.)]×10+[amount of blast furnaceslag-released water sprayed (wt.pts.)]+[amount added of substancescontaining sulfur and/or compounds of sulfur having a valence less than6 (wt.pts.)]×500  (2)

In equation (2), parts by weight (wt.pts.) are all relative to one partby weight of the chromium oxide-containing substances being processed,or to one mg/liter of Cr⁶⁺ having been released from the processedchromium oxide-containing substances, and all the amounts in (2) areproportional to the amount of the chromium oxide-containing substancesbeing processed and that of Cr⁶⁺ having been released from the processedchromium oxide-containing substances.

If A in (1) is less than 0.1, outside the defined range, the release ofCr⁶⁺ from the chromium oxide-containing substances processed is notinhibited sufficiently. On the other hand, even if A is more than 10,also outside the defined range, the saturated effect of the reducingagents used for preventing the release of Cr⁶⁺ from the processedsubstances is not enhanced, and the use of such large amounts of thereducing agents is merely wasteful and hence uneconomical.

The amount of water to be sprayed over the mixture comprising chromiumoxide-containing substances and non-aged, gradually-cooled blast furnaceslag is not specifically defined, but is preferably such that thechromium oxide-containing substances in the mixture may be saturatedwith the water that has been sprayed thereover.

This is because, when chromium oxides are reduced with non-aged,gradually-cooled blast furnace slag or the like, the sulfur and thecompounds of sulfur having a valence less than 6, which have beenreleased from the slag or the like to dissolve in water, act to reduceCr⁶⁺ existing in the chromium oxide-containing substances or Cr⁶⁺ thathas been released from the substances to dissolve in water.

[2] In the embodiment of immersing chromium oxide-containing substancesin reducing baths:

When chromium oxide-containing substances are immersed blast furnaceslag-released water, the reducing sulfur concentration in the water ispreferably greater than 0.03% by weight.

This is because, if chromium oxide-containing substances are immersed inblast furnace slag-released water having a reducing sulfur concentrationof not more than 0.03% by weight, Cr⁶⁺ in the substances is difficult tocompletely reduce within a short period of time.

The reducing sulfur concentration in blast furnace slag-released wateras referred to herein is obtained by subtracting the sulfur content ofSO₄ ²⁻ from the overall sulfur concentration in the water.

(III) Preferred modes of processing chromium oxide-containingsubstances:

In the embodiment of the invention mentioned hereinabove, chromiumoxide-containing substances are mixed with slag that contains sulfurand/or compounds of sulfur having a valence less than 6, such asnon-aged, gradually-cooled blast furnace slag or the like, and then keptin an air atmosphere.

Also in the other embodiments of the invention where chromiumoxide-containing substances are reduced by themselves or after havingbeen mixed with slag that contains sulfur and/or compounds of sulfurhaving a valence less than 6 optionally along with substances thatcontain sulfur and/or compounds of sulfur having a valence less than 6,the reduction system is preferably processed in the intended mannerwhile kept in an air atmosphere.

This is because the method of keeping the reduction system in an airatmosphere is the least expensive.

To keep the reduction system in an air atmosphere, for example, it isdesirable that the substances to be processed are piled up in heaps in ayard equipped with drainage.

The volume of each heap is not specifically determined and may vary froma few m³ to tens of thousands of m³.

Each heap is preferably not high. This is because too high heaps have areduced surface area, and water sprayed thereover or rain falling therewould find it difficult to spread entirely all through the heaps,resulting in that the reduction of chromium oxides in those heaps takesa long time.

In the embodiment of the invention mentioned hereinabove, where steam isapplied to a mixture comprising chromium oxide-containing substances,substances that contain sulfur and/or compounds of sulfur having avalence less than 6, and slag that contains sulfur and/or compounds ofsulfur having a valence less than 6, preferably employed is any mode of<1> spraying steam over the heaps of the mixture, <2> introducing steaminto the heaps of the mixture, or <3> introducing steam into the heapsof the mixture from below, or even any others.

The temperature of the steam to be applied to the mixture is notspecifically defined.

Where steam is applied to the mixture in air, its temperature isgenerally 100° C. However, where steam is introduced into the heaps ofthe mixture under high pressure, the boiling point of water will behigher than 100° C. as the inside of the heaps is under increasedpressure, resulting in that the temperature of the steam that has beenapplied to those heaps will increase, depending on the increasedpressure. Where steam is introduced under high pressure into the mixturein a closed container, the temperature of the steam applied alsoincreases, depending on the increased pressure, as the boiling point ofwater is higher than 100° C. under increased pressure. For example, thetemperature of steam reaches about 180° C. under 10 atmospheres.

In those embodiments, the steam to be applied to the reduction system isnot limited only to water vapor, but may be combined with any othergasses, such as air, N₂ and the like.

In the embodiments of the invention mentioned hereinabove, <1> blastfurnace slag-released water, <2> slag containing sulfur and/or compoundsof sulfur having a valence less than 6, such as non-aged,gradually-cooled blast furnace slag, etc., <3> elementalsulfur-containing substances, and <4> sulfur compounds with a valenceless than 6 may be combined with divalent iron-containing substanceshaving the ability to reduce chromium oxides such as those mentionedhereinabove, and any other reducing agents such as activated charcoaland the like.

The invention is applicable not only to stainless steel slag, chromiumslag, molten slag of industrial waste, sewage sludge, molten slag ofsewage sludge and the like slag, but also to any other substances thatmay release Cr⁶⁺, thereby preventing the release of Cr⁶⁺ from thosesubstances.

Chromium oxide-containing substances to be processed according to theinvention may be pre-treated with steam.

When slag that releases Cr⁶⁺ in an amount of 10 mg/liter or more in themetal release test as set forth in the Notification No. 46 of theEnvironment Agency of Japan, or slag having a low degree of porosity isprocessed according to the method of spraying blast furnaceslag-released water over the slag or according to the method ofimmersing the slag in blast furnace slag-released water, it takes a longtime to safely stabilize the slag.

On the other hand, when the slag of that type is processed according tothe method of applying steam to the slag as mixed with gradually-cooledblast furnace slag, it may be stabilized within a short period of timebut its volume increases as this is mixed with gradually-cooled blastfurnace slag.

Taking those problems into consideration, the present inventors havefurther studied and made various experiments, and, as a result, havefound that, when steam is previously applied to Cr⁶⁺-containing slag(chromium oxide-containing substances), Cr⁶⁺ in the slag can be renderedeasily reducible, and that, when the thus pre-treated, Cr⁶⁺-containingslag is reduced with blast furnace slag-released water, it can bestabilized within a shorter period of time thereby completely to preventthe release of Cr⁶⁺ from the thus-stabilized slag. In other words, wehave found that the pre-treatment with steam shortens the processingtime and simplifies the reduction process itself, and that the methodcomprising the pre-treatment with steam is industrially advantageous andeconomical.

Specifically, in those preferred embodiments, steam is previouslyapplied to chromium oxide-containing substances, and then the thuspre-treated substances are reduced in the manner mentioned hereinabove.For example, blast furnace slag-released water is sprayed over thepre-treated, chromium oxide-containing substances; or the pre-treated,chromium oxide-containing substances are immersed in blast furnaceslag-released water.

These embodiments will be described in more detail hereinafter.

The present inventors have studied and made various experiments in orderto find out an industrial, simple, economical and effective methodcapable of processing slag that releases Cr⁶⁺ in an amount of 10mg/liter or more in the metal release test as set forth in theNotification No. 46 of the Environment Agency of Japan, or slag having alow degree of porosity, within a short period of time without increasingthe volume of the processed slag, thereby completely to prevent therelease of Cr⁶⁺ from the thus-processed slag.

As a result, we have found that, when steam at a high temperature isapplied to the slag of that type, Cr⁶⁺ existing in the slag can be madeeasily releasable or easily reducible. We have further found that, whenblast furnace slag-released water is sprayed over the thus pre-treatedslag or when the thus pre-treated slag is immersed in blast furnaceslag-released water, then the slag can be efficiently and economicallyprocessed within a short period of time without increasing the volume ofthe processed slag, whereby Cr⁶⁺ in the slag is reduced and completelystabilized.

Specifically, according to those preferred embodiments of the invention,where steam is previously applied to chromium oxide-containing slag(chromium oxide-containing substances) and then blast furnaceslag-released water is sprayed over the thus pre-steamed slag, or wheresteam is previously applied to chromium oxide-containing slag (chromiumoxide-containing substances) and then the thus pre-steamed slag isimmersed in blast furnace slag-released water, Cr⁶⁺ in the slag can bereduced within a short period of time without increasing the volume ofthe processed slag. Accordingly, those embodiments of the invention areextremely economical.

FIG. 1 shows the relationship between the time of steaming stainlesssteel slag, which is one example of chromium oxide-containingsubstances, and the amount of Cr⁶⁺ released from the thus-steamed slag.

Briefly, steam at 100° C. was applied to stainless steel slag, fromwhich Cr⁶⁺ is released, for a predetermined period of time, andthereafter the thus-steamed slag was subjected to the metal release testas set forth in the Notification No. 46 of the Environment Agency ofJapan, and the amount of Cr⁶⁺ released from the slag was measured. Thedata obtained are plotted relative to the steaming time in FIG. 1.

The slag used in the test had a grain size of from 13.2 to 26.5 mm, andwas not ground prior to the test.

As in FIG. 1, the amount of Cr⁶⁺ released from the slag was greaterafter the steaming time of 24 hours than that before the steaming at100° C.

Since steam does not have the ability to oxidize chromium, the test dataobtained herein suggest that the steaming at 100° C. makes Cr⁶⁺ in theslag easily releasable, or that is, easily reducible.

This is because the capillary action of steam applied to the slag willmake Cr⁶⁺ existing in the depth of the slag move to and condense in thesurface layer of the slag.

Where steam at 100° C. was applied to the slag for a period of timelonger than 24 hours, the amount of Cr⁶⁺ released from the slagdecreased. This is because Cr⁶⁺ that had been made easily releasablefrom the slag by the initial steaming was dissolved by the successivesteaming and removed.

Accordingly, Cr⁶⁺ could be completely removed from the slag bycontinuous steaming for an extremely long period of time, which,however, is not practical.

On the basis of those findings noted above, the present inventors havefound a preferred embodiment of the invention wherein Cr⁶⁺-containingslag is previously steamed thereby to make Cr⁶⁺ in the slag easilyreleasable or easily reducible, and thereafter the thus-steamed slag isthen reduced with blast furnace slag-released water, for example, waterthat has been sprayed over non-aged, gradually-cooled blast furnaceslag.

The steam to be used in this embodiment is preferably high-temperaturesteam, but its temperature is not specifically defined.

This is because, when steam is applied to Cr⁶⁺-containing slag or thelike in air, its temperature is generally 100° C., but when steam isintroduced under high pressure into a closed container containingtherein Cr⁶⁺-containing slag or the like, the boiling point of waterwill be higher than 100° C. as the inside of the container is underincreased pressure, resulting in that the temperature of the steamhaving been introduced into the container increases, depending on theincreased pressure.

In this embodiment, the steam to be applied to the reduction system isnot limited to water vapor only, but may be combined with any othergases, such as air, N₂ and the like.

The steaming time is not also specifically defined.

Depending on the grain size and the porosity of the slag to bepre-steamed, the steaming time within which Cr⁶⁺ existing in the slag ismade releasable or reducible may be determined. In most cases, thesteaming time may fall between 1 and 120 hours.

The blast furnace slag for use in this embodiment of the invention ispreferably non-aged, gradually-cooled blast furnace slag that colorswater in the color identification test defined in JIS A5015, Appendix 1.As the blast furnace slag-released water for use therein, in general,preferred is water that has been sprayed over non-aged, gradually-cooledblast furnace slag that has been spontaneously aged for less than 3months.

For example, preferably used is water that has been sprayed over hotslag that has just been discharged from blast furnaces.

The effectiveness of the blast furnace slag-released water to reduceCr⁶⁺ into Cr³⁺ is due to the oxidation of the reducing sulfur ofdifferent types (S²⁻, S⁰, S₂O₃ ²⁻) that exists in the water.

In this embodiment of the invention, the blast furnace slag-releasedwater to be used may be combined with any other reducing agents havingthe ability to reduce Cr⁶⁺.

For example, ferrous sulfate may be dissolved in the blast furnaceslag-released water to be used therein.

In this embodiment, the amount of the blast furnace slag-released waterto be sprayed over the slag to be reduced, or the time for which theslag to be reduced is immersed in the blast furnace slag-released water,and the ratio of the slag to be reduced to the blast furnaceslag-released water in which the slag is immersed are not specificallydefined.

The preferred values of those parameters may be suitably determined,depending on the grain size, the porosity and the Cr⁶⁺ content of theslag to be reduced.

The present invention is especially effectively applied to slag that mayrelease a large amount, for example, 10 mg/liter or more of Cr⁶⁺, which,however, is not limitative. Needlesstosay, the invention is alsoeffectively applicable to any and every slag that may release a smallamount of Cr⁶⁺.

The invention is applicable not only to Cr⁶⁺-containing stainless steelslag, chromium slag, industrial wastes, slag of molten wastes and thelike, but also to any other substances that may release Cr⁶⁺, therebypreventing the release of Cr⁶⁺ from those substances.

As its second aspect, the present invention provides a method of safelyrecycling chromium oxide-containing substances such as chromiumoxide-containing slag.

The present inventors have assiduously studied the problems in the priorart of recycling chromium oxide-containing substances noted hereinabove,and, as a result, have found that, when chromium oxide-containingsubstances are, after having been reduced, mixed with a predeterminedamount of sulfur-containing slag, the release of Cr⁶⁺ from the resultingmixture is completely prevented, and the mixture can be effectively usedin roadbeds, fillers for use in civil engineering reclamation works,temporary works, and the like.

The second aspect of the invention will be described in detailhereinafter.

The method of reducing chromium oxide-containing substances for use inthe second aspect of the invention is not specifically defined, providedthat it is to reduce Cr⁶⁺ existing in chromium oxide-containingsubstances, but is preferably such that the amount of Cr⁶⁺ released fromthe reduced chromium oxide-containing substances, when measuredaccording to the metal release test as set forth in the Notification No.46 of the Environment Agency of Japan, may be not greater than 0.05mg/liter.

As the reducing method, any of the conventional reducing methodsmentioned hereinabove may be employed, but more referred is the methodof the first aspect of the invention for reducing chromium oxides notedabove.

In the second aspect of the invention, chromium oxide-containingsubstances are, after having been reduced according to the method notedabove, mixed with sulfur-containing slag. In the resulting mixture, therelease of Cr⁶⁺ from the chromium oxide-containing substances iscompletely prevented, and the mixture is effectively useable inroadbeds, fillers for use in civil engineering reclamation works,temporary works, civil engineering and construction materials, and thelike.

As the sulfur-containing slag for use in the second aspect of theinvention, preferably used is gradually-cooled and aged blast furnaceslag, slag as discharged in the pre-treatment of molten pig iron, theirmixture, and the like.

The amount of the sulfur-containing slag to be added to the reducedchromium oxide-containing substances is preferably from 0.1 to 90 partsby weight relative to 100 parts by weight of the reduced chromiumoxide-containing substances.

When the amount of the sulfur-containing slag added is less than 0.1part by weight, the complete prevention of the release of Cr⁶⁺ from thechromium oxide-containing substances will be difficult if the reductionof the chromium oxide-containing substances is incomplete. On the otherhand, when the amount of the sulfur-containing slag added is greaterthan 90 parts by weight, the energy necessary for mixing them will belarger as the amount of the sulfur-containing slag to be handled islarger. Therefore, the addition of such a large amount ofsulfur-containing slag is unfavorable from the economical viewpoint.

Regarding the sulfur content of the sulfur-containing slag to be used inthe invention, the overall amount of sulfur in the slag, which is thetotal of elemental sulfur and all sulfur compounds therein, ispreferably not less than 0.1% by weight; more preferably, the total ofelemental sulfur and compounds of sulfur having a valence less than 6 inthe slag is not less than 0.2% by weight.

The reason why adding sulfur-containing slag to reduced chromiumoxide-containing substances ensures the complete prevention of therelease of Cr⁶⁺ from the chromium oxide-containing substances is becausethe sulfur component existing in the sulfur-containing slag willdissolve in water thereby acting to reduce Cr⁶⁺.

Specifically, it is believed that S⁰, S₂O₃ ²⁻ and SO₄ ²⁻ in thesulfur-containing slag, such as gradually-cooled and aged blast furnaceslag, slag as discharged in the pre-treatment of molten pig iron or thelike, dissolve in water, whereupon S⁰ and S₂O₃ ²⁻ are oxidized to SO₄ ²⁻while reducing Cr⁶⁺.

In the invention, the sulfur-containing slag is contacted with thechromium oxide-containing substances in the presence of water, whereuponthe slag reduces chromium to a stable chromium compound such as Cr(OH)₃or the like, resulting in that the release of chromium oxides from thechromium oxide-containing substances is completely prevented.

In the invention, the sulfur-containing slag to be added may be combinedwith any other reducing agents having the ability to reduce chromiumcompounds, such as iron sulfate, activated charcoal and the like.

The invention will be described more concretely with reference to thefollowing Examples, which, however, are not intended to restrict thescope of the invention.

In the following Examples, the amount of Cr⁶⁺ released from samples wasmeasured according to the metal release test as described in theNotification No. 46 of the Environment Agency of Japan.

EXAMPLE A1

As chromium oxide-containing substances, herein used were stainlesssteel slag, chromium slag as discharged during the production of sodiumbichromate, and sewage sludge.

The stainless steel slag is produced during reduction in the course ofrefining stainless steel.

As sulfur-containing substances, used herein were non-aged,gradually-cooled blast furnace slag, gradually-cooled blast furnace slagspontaneously aged for 6 months, and blast furnace slag-released water.

The non-aged, gradually-cooled blast furnace slag is one produced 1 weekafter cooling and grinding blast furnace slag.

The blast furnace slag-released water is water that has been sprayedover hot blast furnace slag.

The reducing sulfur concentration in the blast furnace slag-releasedwater used herein was 0.05% by weight.

Table 4 shows the chemical compositions of the samples of non-aged,gradually-cooled blast furnace slag, stainless steel slag, chromium slagand sewage sludge tested herein, and the amount of Cr⁶⁺ released fromeach sample.

In Table 4, the amount of Cr⁶⁺ released from each sample of stainlesssteel slag, chromium slag and sewage sludge was 1.0 mg/liter, 9.7mg/liter and 0.80 mg/liter, respectively.

Those chromium oxide-containing substances (samples to be reduced) wereprocessed under the different conditions shown in Table 5.

The amount of the non-aged, gradually-cooled blast furnace slag added,which is in Table 5, is in terms of % by weight relative to 100% byweight of each sample to be reduced with it.

In Table 5, there are shown the experimental data obtained along withthe conditions employed in the experiments that were conducted.

The sample to be reduced and the non-aged, gradually-cooled blastfurnace slag were mixed uniformly.

Each heap of samples was kept in an air atmosphere and weighed about 100tons and had a height of 2 m.

As seen in Table 5, the amount of Cr⁶⁺ released from comparative samplesA1 to A4 which had been kept in an air atmosphere or had been steamed at100° C., without adding non-aged, gradually-cooled blast furnace slagthereto, was not lower than the environmentally acceptable standardvalue of 0.05 mg/liter. This is because no reducing agent was added tothose comparative samples.

The amount of Cr⁶⁺ released from comparative sample A5, to which hadbeen added gradually-cooled blast furnace slag and spontaneously agedfor 6 months and which had been kept in an air atmosphere, was also nolower than 0.05 mg/liter.

As to comparative sample A6, stainless steel slag, which was kept in anair atmosphere while spraying thereover city water of the same weight asthe slag, there was found no significant difference in the amount ofCr⁶⁺ released from it before and after the treatment.

In contrast to those comparative samples, samples A1 to A16 of theinvention were processed as follows: [1] Each sample was mixed withnon-aged, gradually-cooled blast furnace slag, and then kept in an airatmosphere (samples A1 to A4, A10 and A14 of the invention). [2] Eachsample was mixed with non-aged, gradually-cooled blast furnace slag, andthen kept in an air atmosphere while either one or both of blast furnaceslag-released water and ordinary water was sprayed thereover (sample A7,A8, A9, A12 and A16 of the invention). [3] Each sample was immersed inblast furnace-released water (samples A6, A11 and A15 of the invention).[4] Each sample was mixed with non-aged, gradually-cooled blast furnaceslag, and high-temperature steam was applied thereto (samples A5 and A13of the invention). As a result, the amount of Cr⁶⁺ released from eachsample of the invention thus processed was lower than the environmentalstandard value of 0.05 mg/liter.

As for sample A17, concerning stainless steel slag of the invention,over which was sprayed blast furnace slag-released water of the sameweight as that of the slag, the amount of Cr⁶⁺ released from thethus-processed slag was lower than the environmentally acceptablestandard value.

Although the samples, stainless steel slag, chromium slag and sewagesludge, which had been reduced as described herein according to theinvention, were kept in an air atmosphere for 1 year, they released noCr⁶⁺.

TABLE 4 Cr⁶⁺ Al₂O₃ Total Fe Total Cr Released CaO (wt. %) SiO₂ (wt. %)(wt. %) MgO (wt. %) Total S (wt. %) (wt. %) (wt. %) Water (wt. %)(mg/liter)¹⁾ Non-aged, 42.4 34.2 13.8 7.2 1.09 0.53 — — — gradually-cooled blast furnace slag Stainless 55.6 28.7 4.83 5.0 0.03 2.1 0.6 —1.0 steel slag Chromium 26.8 25.7 15.2 9.8 0.82 7.0 5.2 — 9.7 slagSewage 1.2 13.3 4.9 0.5 0.6 1.2 0.3 75 0.8 sludge Note: 1) Metal releasetest in the Notification No. 46 of the Environment Agency of Japan.

TABLE 5-1 Non-aged, Gradually- cooled Blast Furnace Sample ProcessedSlag Cr⁶⁺ Released Grain Amount Grain (mg/liter)¹⁾ Size Added SizeBefore After Type (mm) (wt. %) (mm) Processing Method A²⁾ TreatmentTreatment Comparative stainless ˜25 0 — Kept in air atmosphere for 3months. 0 1.0 0.98 Sample A1 steel slag Comparative chromium ˜25 0 —Kept in air atmosphere for 3 months. 0 9.7 9.4 Sample A2 slagComparative sewage  ˜5 0 — Kept in air atmosphere for 3 months. 0 0.80.79 Sample A3 sludge Comparative stainless ˜25 0 — Steamed at 100° C.for 24 hours. 0 1.0 0.60 Sample A4 steel slag Comparative stainless ˜250 — 50 wt. % of gradually-cooled blast furnace slag as 5.0 1.0 0.52Sample A5 steel slag spontaneously aged for 6 months (grain size: ˜25mm) was added, and kept in air atmosphere for 3 months. Comparativestainless ˜25 0 — Sprayed with city water of the same weight as that ofthe 0 1.0 0.97 Sample A6 steel slag sample, for 3 months.³⁾ Sample A1 ofstainless ˜40 50  ˜40 Kept in air atmosphere for 3 months. 5.0 1.0 <0.05the Invention steel slag Sample A2 of stainless ˜25 50  ˜25 Kept in airatmosphere for 3 months. 5.0 1.0 <0.05 the Invention steel slag SampleA3 of stainless ˜25 25  ˜25 Kept in air atmosphere for 3 months. 2.5 1.0<0.05 the Invention steel slag Sample A4 of stainless ˜25 50  ˜25 Keptin air atmosphere for 1 month. 5.0 1.0 <0.05 the Invention steel slagSample A5 of stainless ˜25 5 ˜25 Steamed at 100° C. for 24 hours. 0.51.0 <0.05 the Invention steel slag Sample A6 of stainless ˜25 0 —Immersed in blast furnace slag-released water 0 1.0 <0.05 the Inventionsteel slag of the same weight as that of the sample for 1 week. SampleA7 of stainless ˜25 5 ˜25 Kept in air atmosphere for 1 month, whilebeing 0.6 1.0 <0.05 the Invention steel slag sprayed with blast furnaceslag-released water of 0.1 times the weight of the sample.⁴⁾ Notes:¹⁾Metal release test in the Notification No. 46 of the EnvironmentAgency of Japan. ²⁾A in formula (1). ³⁾While kept in an air atmosphere,this was sprayed 10 times with the determined amount of city water asdivided into 10 portions. ⁴⁾The reducing sulfur concentration in theblast furnace slag-released water used was 0.50% by weight. While beingkept in an air atmosphere, the sample was sprayed twice with thedetermined amount of the water as divided into two portions.

TABLE 5-2 Non-aged, Gradually-cooled Cr⁶⁺ Released Sample ProcessedBlast Furnace Slag (mg/liter)¹⁾ Grain Amount Grain Before After SizeAdded Size Treat- Treat- Type (mm) (wt. %) (mm) Processing Method A²⁾ment ment Sample A8 of stainless ˜25  10 ˜25 Kept in air atmosphere for3 months, while 1.0 1.0 <0.05 the Invention steel slag being sprayedwith water of the same weight as that of the sample.³⁾ Sample A9 ofstainless ˜25  7 ˜25 Kept in air atmosphere for 3 months, while 0.8 1.0<0.05 the Invention steel slag being sprayed with blast furnace slag-released water of 0.1 times the weight of the sample⁴⁾ and with water of0.5 times the weight of the sample.³⁾ Sample A10 chromium ˜25 300 ˜25Kept in air atmosphere for 3 months. 3.1 9.7 <0.05 of the slag InventionSample A11 chromium ˜25  0 — Immersed in blast furnace slag-released 09.7 <0.05 of the slag water of 2 times the weight of the sample,Invention for 1 week. Sample A12 chromium ˜25 100 ˜25 Kept in airatmosphere for 1 month, while 1.1 9.7 <0.05 of the slag being sprayedwith blast furnace slag- Invention released water of the same weight asthat of the sample.⁴⁾ Sample A13 chromium ˜25  20 ˜25 Steamed at 100° C.for 72 hours. 0.2 9.7 <0.05 of the slag Invention Sample A14 sewage  ˜5 50 ˜25 Kept in air atmosphere for 3 months. 6.3 0.8 <0.05 of the sludgeInvention Sample A15 sewage  ˜5  0 — Immersed in blast furnaceslag-released 0 0.8 <0.05 of the sludge water of the same weight as thatof the Invention sample, for 1 week. Sample A16 sewage  ˜5  5 ˜25 Keptin air atmosphere for 3 months, while 0.8 0.8 <0.05 of the sludge beingsprayed with blast furnace slag- Invention released water of 0.1 timesthe weight of the slag 4) Sample A17 stainless ˜25  0 — Sprayed withblast furnace slag-released 0.1 1.0 <0.05 of the steel slag water of 1.0times the weight of the sample Invention for 3 months.³⁾ Notes: ¹⁾Metalrelease test in the Notification No. 46 of the Environment Agency ofJapan. ²⁾A in formula (1). ³⁾While kept in an air atmosphere, this wassprayed twice with the determined amount of water as divided into 2portions. ⁴⁾The reducing sulfur concentration in the blast furnaceslag-released water used was 0.50% by weight. While being kept in an airatmosphere, the sample was sprayed twice with the determined amount ofthe water as divided into two portions.

EXAMPLE A2

As chromium oxide-containing substances, there were used hereinstainless steel slag, chromium slag discharged during the production ofsodium bichromate, slag of molten sewage sludge, and stainless steelslag-adhered refractory waste.

Table 6 shows the chemical compositions of the samples of chromiumoxide-containing substances tested herein, the porosity of each sample,and the amount of Cr⁶⁺ released from each sample.

As in Table 6, the amount of Cr⁶⁺ released from each sample of stainlesssteel slag A, stainless steel slag B, stainless steel slag C, chromiumslag, slag of molten sewage sludge, and stainless steel slag-adheredrefractory waste was 6.50 mg/liter, 32.7 mg/liter, 13.2 mg/liter, 25.3mg/liter, 0.8 mg/liter and 0.12 mg/liter, respectively.

The porosity of each sample of stainless steel slag A, stainless steelslag B, stainless steel slag C, chromium slag, and slag of molten sewagesludge was 18%, 25%, 4%, 10% and 8%, respectively.

Stainless steel slag is generally discharged from a converter, afterhaving been reduced. However, the samples of stainless steel slag usedherein were not reduced prior to being discharged.

As sulfur-containing substances, used herein were non-aged,gradually-cooled blast furnace slag, elemental sulfur, and sinter.

The non-aged, gradually-cooled blast furnace slag was sampled within 1week after having been cooled and ground.

Those slag samples to be reduced were steamed under various conditionsshown in Table 7.

In Table 7 are shown the experimental data obtained long with theconditions employed.

As for comparative sample A8, stainless steel slag A, in Table 7, towhich was added 10% by weight of non-aged, gradually-cooled blastfurnace slag and which was steamed for 2 days, the amount of Cr⁶⁺released from the processed slag A was reduced, but still was not lessthan 0.05 mg/liter.

As for comparative sample A9, stainless steel slag B, to which was added10% by weight of non-aged, gradually-cooled blast furnace slag and whichwas steamed for 3 days, the amount of Cr⁶⁺ released from the processedslag B was reduced, but still was not less than 0.05 mg/liter.

As opposed to those comparative samples, the amount of Cr⁶⁺ releasedfrom samples A18 to A26 of the invention, to which had been added asmall amount of a sulfur-containing substance having a high sulfurcontent and which had been steamed for 3 days or shorter, was less than0.05 mg/liter.

As for comparative sample A10, stainless steel slag C, to which wasadded 20% by weight of non-aged, gradually-cooled blast furnace slag andwhich was steamed for 3 days, the amount of Cr⁶⁺ released from theprocessed slag C was reduced, but still was not less than 0.05 mg/liter.As opposed to this comparative sample, the amount of Cr⁶⁺ released fromsample A25 of the invention, to which had been added 5% by weight ofnon-aged, gradually-cooled blast furnace slag and 1% by weight of sulfurand which had been steamed for 3 days, was less than 0.05 mg/liter.

TABLE 6 Cr⁶⁺ Total Fe Total Cr Released Cao (wt. %) SiO₂ (wt. %) Al₂O₃(wt. %) MgO (wt. %) Total S (wt. %) (wt. %) (wt. %) Porosity (%)(mg/liter)¹⁾ Stainless 57.0 20.4 3.8 4.9 0.03 3.6 4.5 18 6.50 steel slagA Stainless 61.3 17.5 2.8 2.6 0.03 4.3 6.2 25 32.7 steel slag BStainless 58.3 18.8 3.3 2.5 0.03 4.2 5.5  4 13.2 steel slag C Chromium28.5 24.9 13.8 9.5 0.91 6.5 7.4 10 25.3 slag Slag of 10.5 47.5 20.2 3.02.5 5.2 1.6  8 0.80 molten sewage sludge Stainless 9.5 13.2 24.4 37.10.08 4.5 0.3 — 0.12 steel slag- adhered refractory waste Note: ¹⁾Metalrelease test in the Notification No. 46 of the Environment Agency ofJapan.

TABLE 7-1 Non-aged, Gradually-cooled Cr⁶⁺ Released Sample ProcessedBlast Furnace Slag (mg/liter)¹⁾ Grain Size Amount Grain Size BeforeAfter Type (mm) Added (wt. %) (mm) Processing Method A²⁾ TreatmentTreatment Comparative stainless ˜40  0 — Steamed at 100° C. for 3 days.0 6.5 5.20 Sample 7 steel slag A Comparative stainless ˜40 10 ˜25Steamed at 100° C. for 2 days. 0.15 6.5 0.25 Sample 8 steel slag AComparative stainless ˜25 10 ˜25 Steamed at 100° C. for 3 days. 0.0332.7 1.31 Sample 9 steel slag B Comparative stainless ˜40 20 ˜25 Steamedat 100° C. for 3 days. 0.15 13.2 1.52 Sample 10 steel slag C Comparativechromium ˜25 10 ˜25 Steamed at 100° C. for 3 days. 0.04 25.3 1.95 Sample11 slag Notes: ¹⁾Metal release test in the Notification No. 46 of theEnvironment Agency of Japan. ²⁾A in formula (1).

TABLE 7-2 Non-aged, Gradually-cooled Cr⁶⁺ Released Sample ProcessedBlast Furnace Slag Additive (mg/liter)¹⁾ Grain Amount Grain AmountBefore After Size Added Size Added Treat- Treat- Type (mm) (wt. %) (mm)Type (wt. %) Processing Method A²⁾ ment ment Sample A18 of stainlesssteel ˜40 0 — sulfur 0.5 Steamed at 100° C. for 2 days. 0.38 6.50 <0.05the Invention slag A Sample A19 of stainless steel ˜40 0 — sinter⁵⁾ 2Steamed at 100° C. for 2 days. 1.54 6.50 <0.05 the Invention slag ASample A20 of stainless steel ˜40 0 — sulfur 2 Steamed at 100° C. for 3days. 0.31 32.7 <0.05 the Invention slag B Sample A21 of chromium slag˜25 0 — sulfur 2 Steamed at 100° C. for 3 days. 0.40 25.3 <0.05 theInvention Sample A22 of slag of molten ˜25 0 — sulfur 0.5 Steamed at100° C. for 2 days. 3.13 0.80 <0.05 the Invention sewage sludge SampleA23 of stainless steel ˜40 0 — sulfur 0.5 Steamed at 100° C. for 1 day. 20.8 0.12 <0.05 the Invention slag-adhered refractory waste Sample A24of stainless steel ˜25 5 ˜25 sulfur 1 Steamed at 100° C. for 3 days.0.17 32.7 <0.05 the Invention slag B Sample A25 of stainless steel ˜40 5˜25 sulfur 1 Steamed at 100° C. for 3 days. 0.42 13.2 <0.05 theInvention slag C Sample A26 of chromium slag ˜25 5 ˜25 sulfur 1 Steamedat 100° C. for 3 days. 0.22 25.3 <0.05 the Invention Notes: ¹⁾Metalrelease test in the Notification No. 46 of the Environment Agency ofJapan. ²⁾A in formula (1). ⁵⁾Sinter as purified from a hot spring fromKusazu Hot Spring Resort, Gunma-ken, Japan (this has a total sulfurcontent of 27 wt. %).

EXAMPLE A3

The same chromium oxide-containing substances as in Example A2 wereprocessed under different immersion conditions shown in Table 8.

As sulfur-containing substances, there were used herein blast furnaceslag-released water, elementary sulfur, sinter, hydrogen sulfide, andsodium thiosulfate; and as a divalent iron-containing compound, therewas used ferrous sulfate.

In Table 8, there are shown the experimental data obtained along withthe conditions employed.

As for comparative samples A12 to A15 in Table 8, which were immersed inblast furnace slag-released water having a reducing sulfur concentrationof 0.03% by weight for 3 to 5 days, the amount of Cr⁶⁺ released from theprocessed samples was reduced, but still was not less than 0.05mg/liter.

In contrast to those comparative samples, samples A27 to A31 of theinvention were immersed in blast furnace slag-released water, to whichor over which had been added or sprayed any of elemental sulfur, sinter,sodium thiosulfate and hydrogen sulfate (that is, any of elementalsulfur-containing substances or compounds of sulfur having a valenceless than 6) thereby to control the reducing sulfur content of theresulting water, for 5 days or less. As a result, the amount of Cr⁶⁺released from those processed samples of the invention was less than0.05 mg/liter.

As for sample A32 of the invention, it was immersed in blast furnaceslag-released water, to which had been added ferrous sulfate, for 3days. The amount of Cr⁶⁺ released from this sample A32 was also lessthan 0.05 mg/liter.

TABLE 8-1 Non-aged, Gradually- cooled Blast Furnace Cr⁶⁺ Released Slag(mg/liter)¹⁾ Sample Processed Amount Before After Grain Size Added GrainTreat- Treat- Type (mm) (wt. %) Size (mm) Processing Method A²⁾ mentment Comparative stainless steel ˜40 0 — Immersed in blast furnaceslag-released water⁴⁾ of 0 6.50 1.76 Sample A12 slag A 0.5 times theweight of the sample, for 3 days. Comparative stainless steel ˜25 0 —Immersed in blast furnace slag-released water⁴⁾ of 0 32.7 6.13 SampleA13 slag B 3.0 times the weight of the sample, for 3 days. Comparativeslag of molten ˜25 0 — Immersed in blast furnace slag-released water⁴⁾of 0 0.80 0.35 Sample A14 sewage sludge 0.5 times the weight of thesample, for 5 days. Comparative stainless steel ˜40 0 — Immersed inblast furnace slag-released water⁴⁾ of 0 0.12 0.06 Sample A15slag-adhered 0.5 times the weight of the sample, for 3 days. refractorywaste Notes: ¹⁾Metal release test in the Notification No. 46 of theEnvironment Agency of Japan. ²⁾A in formula (1). ³⁾The reducing sulfurconcentration in the blast furnace slag-released water used was 0.3% byweight.

TABLE 8-2 Non-aged, Gradually- cooled Blast Furnace Cr⁶⁺ Released Slag(mg/liter)¹⁾ Sample Processed Amount Grain Before After Grain Size AddedSize Treat- Treat- Type (mm) (wt. %) (mm) Processing Method A²⁾ mentment Sample A27 of stainless ˜40 0 — Immersed in blast furnaceslag-released water⁶⁾ of 0.5 0 6.50 <0.05 the Invention steel slag Atimes the weight of the sample, to which had been added sinter⁷⁾ to makethe water have a reducing sulfur concentration of 1 g/liter, for 3 days.Sample A28 of stainless ˜40 0 — Immersed in blast furnace slag-releasedwater⁶⁾ of 0.5 0 6.50 <0.05 the Invention steel slag A times the weightof the sample, over which had been sprayed hydrogen sulfide to make thewater have a reducing sulfur concentration of 2 g/liter, for 3 days.Sample A29 of stainless ˜25 0 — Immersed in blast furnace slag-releasedwater⁶⁾ of 0.5 0 32.7 <0.05 the Invention steel slag B times the weightof the sample, to which had been added sodium thiosulfate to make thewater have a reducing sulfur concentration of 10 g/liter, for 5 days.Sample A30 of slag of ˜25 0 — Immersed in blast furnace slag-releasedwater⁶⁾ of 0.5 0 0.80 <0.05 the Invention molten times the weight of thesample, to which had been sewage added sulfur to make the water have areducing sulfur sludge concentration of 0.4 g/liter, for 3 days. SampleA31 of stainless ˜40 0 — Immersed in blast furnace slag-released water⁶⁾of 0.4 0 0.12 <0.05 the Invention steel slag- times the weight of thesample, to which had been adhered added sulfur to make the water have areducing sulfur refractory concentration of 0.4 g/liter, for 3 days.waste Sample A32 of stainless ˜40 0 — Immersed in blast furnaceslag-released water⁶⁾ of 0.5 0 6.50 <0.05 the Invention steel slag Atimes the weight of the sample, to which had been added ferrous sulfate,for 3 days. Notes: ¹⁾Metal release test in the Notification No. 46 ofthe Environment Agency of Japan. ²⁾A in formula (1). ⁶⁾The reducingsulfur concentration in the blast furnace slag-released water used was0.03% by weight. ⁷⁾Sinter as purified from a hot spring from Kusazu HotSpring Resort, Gunma-ken, Japan (this has a total sulfur content of 20wt. %).

EXAMPLE B

As chromium oxide-containing substances which are to be reduced herein,there were used stainless steel slag, chromium slag as discharged duringthe production of sodium bichromate, and slag of molten sewage sludge.

The stainless steel slag is one produced in a stainless steel refiningprocess operated under such conditions that the slag discharged containsreleasable Cr⁶⁺.

Table 9 shows the chemical compositions of the samples of stainlesssteel slag, chromium slag and slag of molten sewage sludge testedherein, the porosity of each sample, and the amount of Cr⁶⁺ releasedfrom each sample.

As in Table 9, the amount of Cr⁶⁺ released from each sample of stainlesssteel slag, chromium slag, slag A of molten sewage sludge (porosity:8%), and slag B of molten sewage sludge (porosity: 2%) was 10.5mg/liter, 25.3 mg/liter, 0.8 mg/liter and 0.27 mg/liter, respectively.

Those slag samples to be reduced were processed under the variousconditions shown in Table 10. The amount of Cr⁶⁺ released from each ofthe thus-processed sample was measured.

The data obtained are shown in Table 10.

The amount of each slag sample processed was 100 tons/batch. Blastfurnace slag-released water was used herein, which is water that hasbeen sprayed over hot, non-aged blast furnace slag. This had a totalsulfur concentration of 0.50% by weight.

Steam was introduced into each sample from below, and its temperaturewas 100° C.

As in Table 10, even though non-aged blast furnace slag-released waterwas sprayed over comparative sample B1, stainless steel slag whichreleased 10.5 mg/liter of Cr⁶⁺ prior to the treatment, the amount ofCr⁶⁺ released from the thus-processed sample B1 was still not less thanthe environmentally acceptable standard value of 0.05 mg/liter.

As for comparative samples B2 to B5, stainless steel slag, chromium slagand slag of molten sewage sludge, even though these were immersed innon-aged blast furnace slag-released water for 7 days, the amount ofCr⁶⁺ released from the thus-processed samples was still not less than0.05 mg/liter.

In contrast to those comparative samples, samples B1 to B9 of theinvention were processed by steaming them followed by spraying thereovernon-aged blast furnace slag-released water, or by steaming them followedby immersing them in non-aged blast furnace slag-released water. As aresult, the amount of Cr⁶⁺ released from the thus-processed samples ofthe invention was less than the environmentally acceptable standardvalue of 0.05 mg/liter.

Although samples B1 to B9, stainless steel slag, chromium slag and slagof molten sewage sludge, which had been reduced herein according to theinvention, were kept in air for 1 year, they released no Cr⁶⁺.

TABLE 9 Cr6+ Total Fe Total Cr Released CaO (wt. %) SiO₂ (wt. %) Al₂O₃(wt. %) MgO (wt. %) Total S (wt. %) (wt. %) (wt. %) Porosity (%)(mg/liter)(*) Stainless 59.2 17.3 4.2 3.5 0.03 4.1 5.7 14 10.5 SteelSlag Chromium 28.5 24.9 13.8 9.5 0.91 6.5 7.4 10 25.3 Slag Slag A of10.5 47.5 20.2 3.0 2.5 5.2 1.6 8 0.80 Molten Sewage Sludge Slag B of10.5 47.5 20.2 3.0 2.5 5.2 1.6 2 0.27 Molten Sewage Sludge (*): Metalrelease test in the Notification No. 46 of the Environment Agency ofJapan.

TABLE 10 Cr⁶⁺ Released (mg/liter)¹⁾ Sample Processed Before After GrainSize Treat- Treat- Type (mm) Processing Method ment ment Comparativestainless steel slag ˜25 Sprayed with blast furnace slag-released waterof 0.1 times the weight of the 10.5 2.21 sample B1 sample, at intervalsof 5 days for a total of 10 times. Comparative stainless steel slag ˜25Immersed in blast furnace slag-released water of 5 times the weight ofthe sample, 10.5 0.80 sample B2 for 7 days. Comparative chromium slag˜25 Immersed in blast furnace slag-released water of 5 times the weightof the sample, 25.3 4.36 sample B3 for 7 days. Comparative slag A ofmolten  ˜5 Immersed in blast furnace slag-released water of the sameweight as that of the 0.80 0.37 sample B4 sewage sludge sample, for 7days. Comparative slag B of molten  ˜5 Immersed in blast furnaceslag-released water of the same weight as that of the 0.27 0.10 sampleB5 sewage sludge sample, for 7 days. Comparative stainless steel slag˜25 Steamed at 100° C. for 24 hours. 10.5 14.2 sample B6 Comparativechromium slag ˜25 Steamed at 100° C. for 24 hours. 25.3 32.4 sample B7Comparative slag A of molten  ˜5 Steamed at 100° C. for 24 hours. 0.800.98 sample B8 sewage sludge Comparative slag B of molten  ˜5 Steamed at100° C. for 24 hours. 0.27 0.98 sample B9 sewage sludge Sample B1 ofstainless steel slag ˜25 Steamed at 100° C. for 24 hours, and thensprayed once with blast furnace slag- 10.5 <0.05 the invention releasedwater of 0.1 times the weight of the sample. Sample B2 of stainlesssteel slag ˜25 Steamed at 100° C. for 24 hours, and then immersed inblast furnace slag-released 10.5 <0.05 the Invention water of 0.5 timesthe weight of the sample for 1 day. Sample B3 of stainless steel slag˜40 Steamed at 100° C. for 72 hours, and then immersed in blast furnaceslag-released 10.5 <0.05 the Invention water of 0.5 times the weight ofthe sample for 5 days. Sample B4 of chromium slag ˜25 Steamed at 100° C.for 24 hours, and then sprayed with blast furnace slag- 25.3 <0.05 theInvention released water of 0.1 times the weight of the sample atintervals of 5 days for a total of 3 times. Sample B6 of slag A ofmolten  ˜5 Steamed at 100° C. for 72 hours, and then immersed in blastfurnace slag-released 0.80 <0.05 the Invention sewage sludge water of0.5 times the weight of the sample for 5 days. Sample B7 of slag A ofmolten  ˜5 Steamed at 100° C. for 24 hours, and then sprayed once withblast furnace slag- 0.80 <0.05 the Invention sewage sludge releasedwater of 0.1 times the weight of the sample. Sample B8 of slag B ofmolten  ˜5 Steamed at 100° C. for 72 hours, and then immersed in blastfurnace slag-released 0.27 <0.05 the Invention sewage sludge water of0.5 times the weight of the sample for 5 days. Sample B9 of slag B ofmolten  ˜5 Steamed at 100° C. for 24 hours, and then sprayed once withblast furnace slag- 0.27 <0.05 the Invention sewage sludge releasedwater of 0.1 times the weight of the sample. Notes: ¹⁾Metal release testin the Notification No. 46 of the Enviromnent Agency of Japan. The totalsulfur content of the blast furnace slag-released water used was 0.50%by weight.

EXAMPLE C Comparative Samples C1 and C2

As chromium oxide-containing substances, there were used herein (1)stainless steel slag, and (2) stainless steel slag-adhered refractory.

The stainless steel slag is one produced during reduction in the courseof refining stainless steel.

The amount of Cr⁶⁺ released from these chromium oxide-containingsubstances is as follows:

(1) Stainless steel slag: 10.5 mg/liter

(2) Stainless steel slag-adhered refractory: 0.12 mg/liter

These chromium oxide-containing substances were reduced according to themethod C or D shown in Table 11.

In Table 12, there is shown the rejection rate of the reduced samples,from which the amount of Cr⁶⁺ released was more than 0.05 mg/liter,along with the reducing method employed.

As in Table 12, the rejection rate of the reduced samples of stainlesssteel slag and stainless steel slag-adhered refractory, from which theamount of Cr⁶⁺ released was more than 0.05 mg/liter, namely 0.10%.

Comparative Samples C3 to C6

As chromium oxide-containing substances, there were used herein the samestainless steel slag and stainless steel slag-adhered refractory as incomparative samples C1 and C2, and chromium slag and slag of moltensewage sludge.

The amount of Cr⁶⁺ released from the latter two chromiumoxide-containing substances is as follows:

(1) Chromium slag: 25.3 mg/liter

(2) Slag of molten sewage sludge: 0.80 mg/liter

These chromium oxide-containing substances were reduced according to themethod B, C or D shown in Table 11.

0.05 or 0.09 part by weight of aged, gradually-cooled blast furnaceslag, or slag as discharged in the pre-treatment of molten pig iron, wasadded to 100 parts by weight of each of the thus-reduced chromiumoxide-containing substances, and mixed.

Regarding the sulfur content of the aged, gradually-cooled blast furnaceslag used herein, the total of elementary sulfur and sulfur of sulfurcompounds in the slag was 0.85% by weight, and the total of elementarysulfur and sulfur having a valence less than 6 therein was 0.42% byweight.

Regarding the sulfur content of the slag as discharged in thepre-treatment of molten pig iron, which was also used herein, the totalof elementary sulfur and sulfur of sulfur compounds in the slag was0.40% by weight, and the total of elementary sulfur and sulfur having avalence less than 6 therein was 0.12% by weight.

In Table 12, there is shown the rejection rate of the mixture samples,in which the amount of Cr⁶⁺ released was more than 0.05 mg/liter, alongwith the reducing method employed, and the type and the amount of thesulfur-containing slag added.

As shown in Table 12, when the sulfur-containing slag was added to thereduced chromium oxide-containing substance in an amount of less than0.1 parts by weight, relative to 100 parts by weight of the reducedchromium oxide-containing substance, the rejection rate of the mixturesamples, in which the amount of Cr⁶⁺ released was over 0.05 mg/liter,was 0.05%.

Sample C1 of the Invention

The same stainless steel slag as that in comparative sample C1 wasreduced according to the method C shown in Table 11.

0.1 part by weight of aged, gradually-cooled blast furnace slag wasadded to 100 parts by weight of the thus-reduced stainless steel slag,and mixed.

Regarding the sulfur content of the aged, gradually-cooled blast furnaceslag used herein, the total of elementary sulfur and sulfur of sulfurcompounds in the slag was 0.85% by weight, and the total of elementarysulfur and sulfur having a valence less than 6 therein was 0.42% byweight.

As shown in Table 12, the rejection rate of the mixture samples, fromwhich the amount of Cr⁶⁺ released was more than 0.05 mg/liter, was0.00%.

Samples C2 to C24 of the Invention

As chromium oxide-containing substances, there were used herein (1)stainless steel slag, (2) stainless steel slag-adhered refractory, (3)chromium slag, and (4) slag of molten sewage sludge.

The amount of Cr⁶⁺ released from these chromium oxide-containingsubstances is as follows:

(1) Stainless steel slag: 10.5 mg/liter

(2) Stainless steel slag-adhered refractory: 0.12 mg/liter

(3) Chromium slag: 25.3 mg/liter

(4) Slag of molten sewage sludge: 0.80 mg/liter

These chromium oxide-containing substances were reduced according to themethod A, B, C or D shown in Table 11. From 0.1 to 90 parts by weight ofaged, gradually-cooled blast furnace slag, or slag as discharged in thepre-treatment of molten pig iron, was added to 100 parts by weight ofeach of the thus-reduced chromium oxide-containing substances, andmixed.

Regarding the sulfur content of the aged, gradually-cooled blast furnaceslag used herein, the total of elementary sulfur and sulfur of sulfurcompounds in the slag was 0.85% by weight, and the total of elementarysulfur and sulfur having a valence less than 6 therein was 0.42% byweight.

Regarding the sulfur content of the slag as discharged in thepre-treatment of molten pig iron, which was also used herein, the totalof elementary sulfur and sulfur of sulfur compounds in the slag was0.40% by weight, and the total of elementary sulfur and sulfur having avalence less than 6 therein was 0.12% by weight.

In Table 12, there is shown the rejection rate of the mixture samples,from which the amount of Cr⁶⁺ released was more than 0.05 mg/liter,along with the reducing method employed, and the type and the amount ofthe sulfur-containing slag added.

As shown in Table 12, the rejection rate of the mixture samples, fromwhich the amount of Cr⁶⁺ released was more than 0.05 mg/liter, was0.00%. Comparing the samples of the invention with the comparativesamples, it will be seen that the rejection rate of the former issignificantly lower than that of the latter, namely from 0.05 to 0.10%.The rejection rate as referred to herein is one parameter in recyclingchromium oxide-containing wastes as materials in roadbeds, materials intemporary works, fillers in civil engineering reclamation works and thelike.

In this Example, non-aged, gradually-cooled blast furnace slag, andblast furnace slag-released water were used for reducing the chromiumoxide-containing substances prior to adding the sulfur-containing slagto the reduced substances. In the present invention, however, thereduction of chromium oxide-containing substances is not specificallydefined.

TABLE 11 Reduction of Chromium Oxide-containing Substances beforeAddition thereto of Sulfur-containing Slag A Mixed with non-aged,gradually-cooled blast furnace slag, and then sprayed with water. BImmersed in blast furnace slag-released water for 1 week. C Mixed withnon-aged, gradually-cooled blast furnace slag, and then steamed at 100°C. for 24 hours. D Sprayed with blast furnace slag-released water.

TABLE 12 Reduction before Mixed with Sulfur- Sulfur-containing SlagAdded Rejection containing Amount Added Ratio Sample Processed Slag(*)Type (wt. pts.) (**) (%) (***) Comparative stainless steel slag C none 00.10 Sample C1 Comparative stainless steel slag-adhered refractory Dnone 0 0.10 Sample C2 Comparative stainless steel slag C aged,gradually-cooled blast furnace slag 0.05 0.05 Sample C3 Comparativestainless steel slag-adhered refractory D slag as discharged inpre-treatment of pig iron melt 0.09 0.05 Sample C4 Comparative chromiumslag C aged, gradually-cooled blast furnace slag 0.05 0.05 Sample C5Comparative slag of molten sewage sludge B slag as discharged inpre-treatment of pig iron melt 0.09 0.05 Sample C6 Sample C1 stainlesssteel slag C aged, gradually-cooled blast furnace slag 0.1 0.00 of theInvention Sample C2 stainless steel slag-adhered refractory D slag asdischarged in pre-treatment of pig iron melt 0.1 0.00 of the InventionSample C3 chromium slag C aged, gradually-cooled blast furnace slag 0.10.00 of the Invention Sample C4 slag of molten sewage sludge B slag asdischarged in pre-treatment of pig iron melt 0.1 0.00 of the InventionSample C5 stainless steel slag D aged, gradually-cooled blast furnaceslag 5.0 0.00 of the Invention Sample C6 stainless steel slag-adheredrefractory D slag as discharged in pre-treatment of pig iron melt 5.00.00 of the Invention Sample C7 chromium slag D aged, gradually-cooledblast furnace slag 5.0 0.00 of the Invention Sample C8 slag of moltensewage sludge D slag as discharged in pre-treatment of pig iron melt 5.00.00 of the Invention Sample C9 stainless steel slag B aged,gradually-cooled blast furnace slag 0.1 0.00 of the Invention Sample C10stainless steel slag-adhered refractory B slag as discharged inpre-treatment of pig iron melt 0.1 0.00 of the Invention Sample C11chromium slag B aged, gradually-cooled blast furnace slag 0.1 0.00 ofthe Invention Sample C12 slag of molten sewage sludge A slag asdischarged in pre-treatment of pig iron melt 0.1 0.00 of the InventionSample C13 stainless steel slag B aged, gradually-cooled blast furnaceslag 5.0 0.00 of the Invention Sample C14 stainless steel slag-adheredrefractory B slag as discharged in pre-treatment of pig iron melt 5.00.00 of the Invention Sample C15 chromium slag B aged, gradually-cooledblast furnace slag 5.0 0.00 of the Invention Sample C16 slag of moltensewage sludge B slag as discharged in pre-treatment of pig iron melt 5.00.00 of the Invention Sample C17 stainless steel slag C aged,gradually-cooled blast furnace slag 2.0 0.00 of the Invention Sample C18stainless steel slag-adhered refractory C slag as discharged inpre-treatment of pig iron melt 2.0 0.00 of the Invention Sample C19chromium slag C aged, gradually-cooled blast furnace slag 2.0 0.00 ofthe Invention Sample C20 slag of molten sewage sludge C slag asdischarged in pre-treatment of pig iron melt 2.0 0.00 of the InventionSample C21 stainless steel slag C slag as discharged in pre-treatment ofpig iron melt 90 0.00 of the Invention Sample C22 stainless steelslag-adhered refractory B slag as discharged in pre-treatment of pigiron melt 90 0.00 of the Invention Sample C23 chromium slag C aged,gradually-cooled blast furnace slag 90 0.00 of the Invention Sample C24slag of molten sewage sludge A aged, gradually-cooled blast furnace slag90 0.00 of the Invention Notes: *See Table 11. **Parts by weight,relative to 100 parts by weight of the reduced chromium oxide-containingsubstance. ***Rejection ratio of samples as reduced and mixed withsulfur-containing slag, from which the amount of Cr⁶⁺ released was over0.05 mg/liter.

Although the invention has been described in detail and with referenceto specific embodiments thereof, it will be apparent to one skilled inthe art that various changes and modifications can be made thereinwithout departing from the spirit and scope thereof.

What is claimed is:
 1. A method for processing chromium oxide-containingsubstances, comprising the steps of: mixing and contacting a reducingsubstance having therein at least one member selected from the groupconsisting of sulfur and compounds of sulfur having a valence less than6 with chromium oxide-containing substances to form a resulting mixture;and applying steam to the resulting mixture; wherein the mixing andcontacting of the selected reducing substance with the chromiumoxide-containing substances to form the mixture and the subsequentapplication of steam to the resulting mixture is controlled withoutincreasing the volume of the processed slag, resulting in that therelease of Cr⁶⁺ from the chromium oxide-containing substances in thethus-processed slag is prevented.
 2. The method for processing chromiumoxide-containing substances as claimed in claim 1, wherein slag is thereducing substance.
 3. The method of claim 1, wherein said step ofsteaming is performed at a temperature of up to about 200° C. using avapor.
 4. The method for processing chromium oxide-containing substancesas claimed in claim 2, wherein, said mixture is, prior to the applyingsteam step, further mixed with other reducing substances that containsaid at least one member.
 5. The method for processing chromiumoxide-containing substances as claimed in claim 1, wherein non-aged,gradually-cooled blast furnace slag is the selected reducing substance.6. The method for processing chromium oxide-containing substances asclaimed in claim 1, wherein, prior to said mixing step, steam is appliedto said chromium oxide-containing substances.